Model aircraft motors adapted to be driven by gas evaporated from a reservoir of liquefied carbon dioxide, such as may be provided e.g. by a capsule of the type marketed by the present applicants under the registered trademark SPARKLETS, are already known. However, a major difficulty which arises with existing motors of this type is the progressive fall in gas pressure which occurs as gas flows from the reservoir in order to drive the motor, and which limits the power of the motor to a rather low level. This fall in pressure is a consequence of the cooling of the gas as it attempts to evaporate from the liquid state in the reservoir and to expand during consumption by the motor. This cooling effect becomes worse as one attempts to increase the speed and power of the motor and can even cause formation of ice on the outside of the reservoir vessel. Furthermore the cooling of the gas causes its density to increase with the result that gas consumption is increased undesirably. A further disadvantage of existing motors of this type arises because the gas taken from the reservoir for such existing motors is at or near the condition known as `saturation` with the consequence that, as soon as it is expanded in the motor, it inevitably condenses partly back into its liquid or even its solid state. Apart from the possibility of damage to the motor such condensation also causes a large increase in the specific volume of the working fluid and this requires that the motor should have a high expansion ratio in order to adequately expand the working fluid and so extract its available energy. This in turn leads to the need for an undesirably large motor, or to an undesirably low charge volume (which reduces motor power), or to the need for excessively high rpm in order to secure sufficient power from the motor.
In order to overcome these problems it is proposed in U.S. Pat. No. 4,092,830 to provide a container holding or capable of being charged with a buffer substance in heat conductive relationship with the reservoir or with a passage through which, in operation, the gas is supplied from the reservoir to the motor. By the term `buffer substance` in this specification is meant a substance which undergoes a change in its physical, chemical, crystallographic or other state during operation of the motor, the change of state causing a release of heat to the gas. The choice and effects of suitable buffer substances are discussed in the U.S. Patent mentioned above, the disclosure of which is incorporated herein by reference.
Motors which make use of this buffering technique can conveniently be referred to as `stored energy motors` because their buffer substances effectively store heat energy which is released to the working fluid for conversion into power as the motor runs. It is to this class of motor that the present invention relates, an aim of the invention being to provide a motor in which the efficacy of buffering is improved over those designs of stored energy motors hitherto proposed. The invention will be described hereinafter in terms of a motor for powering model aircraft, although it is to be noted that motors in accordance with the invention may be found equally useful in many other applications, e.g. for driving other toys and models, portable power tools, hedge trimmers, dentists' drills, lawn mowers and the like, and in various light automative applications.